The present invention relates to a fluororesin composition for a sliding member comprising a fluororesin, a phosphate and a filler composed of glass fiber, glass powder, carbon fiber or carbon powder, and a sliding member comprising the fluororesin composition, which has an excellent sliding property especially under conditions of a low speed and a high load (high surface pressure).
A polytetrafluoroethylene (which is abbreviated as "PTFE" hereinafter), which is one of the fluororesins, is not only excellent in heat resistance and chemical resistance but also has a low friction coefficient with a self-lubricating property, so that it is widely used in various fields as a representative of the so-called engineering plastics such as a material for a sliding member such as bearings and gears, and a molded article such as a tube and a valve. However, its wear resistance is not necessarily sufficient, and the deformation (creep) due to a load is large, so that its use is restricted under a high load or a high temperature.
However, a sliding member consisting of this PTFE is inferior in wear resistance and the creep resistance, so that depending on the use of the sliding member, for example, 1 by blending PTFE with a filler such as graphite, molybdenum disulfide, glass fiber or the like, or 2 by impregnating PTFE in and coating PTFE on a porous sintered metal layer integrally formed on a thin steel plate, the above-mentioned drawbacks are solved.
The sliding member of the above-mentioned embodiment 2 is one so-called as a multi-layer sliding member having a thin wall-thickness of the member itself (usually 1.5 to 3.0 mm) and showing a greatly improved load resistance, so that it is suitable for the use under conditions of low speed and high load.
However, on the other hand, in the case where an with a large grasping force (high surface pressure) and the rotation of the opposite member is smoothly supported with by sliding (in other words, a portion in which a clearance between the sliding member and the opposite member is loose) such as, for example, a hinge portion of various doors or a sliding portion of a ball joint for automobiles, creep and the like of the sliding member itself occurs and as a result it is absolutely impossible to withstand repeated use.
As sliding materials for solving the above-mentioned problems, for example, there have been proposed:
(1) a coated sheet as a bearing material comprising a metal net and a fluoropolymer sintered thereto, where intersectional wires of the metal net themselves are melted to be connected with each other by a thermal operation at intersectional portions (Japanese Patent Publication No. 35107-1978 corresponding to U.S. Pat. No. 3,899,227), and
(2) a sliding member comprising as a base body an expanded metal of stainless steel or a phosphorous bronze alloy provided with regular networks, and a lubricating composition containing a polytetrafluoroethylene as a main component with 5 to 30% by weight of a phenol resin subjected to heat-treatment and/or addition polymerization type polyimide resin as a filler, which is filled in the networks of the base member and coated on the base member (Japanese Patent Application Laid-open (KOKAI) No. 79417/1989).
Moreover, as sliding members containing a fluororesin as a main component, there have been hitherto proposed:
(3) a porous structural article having a tetrafluoroethylene resin coating on one surface such that when an unsintered tetrafluoroethylene resin mixture containing a liquid lubricant is placed on the surface of a porous structural article to be coated in a powder state as it is or having been molded beforehand so as to perform rolling and coating between two rolls, a layer, interposed from the surface of the roll on the side of contacting with its back surface to the surface of the roll, is formed of a material which is soft and is allowed to enter into the surface of the porous structural article, and the resultant layer is rolled (Japanese Patent Publication No. 19053/1964),
(4) a lining foil comprising a metal textile, a fluoroplastic, and a material containing a reinforcing material of inorganic fiber, in which a fluoroplastic filled with glass fiber or other inorganic fiber, preferably polytetrafluoroethylene or fluoroethylenepropylene is rolled, extruded, or pressed in the form of powder, paste, or an unsintered subassembly on the metal textile at a processing temperature of a room temperature or a high temperature, and the obtained product is baked at a temperature not less than the melting point of said used fluorine polymer (Japanese Patent Publication No. 23740/1980),
(5) a sliding bearing member made of an expanded metal having a thickness of 0.3 to 0.9 mm which is a wrought aluminum alloy having an elongation at breakage (.delta..sub.5) of 8 to 20% and a Brinell hardness (HB) of 35 to 65 and a matrix containing 5 to 25% by volume of lead, 10 to 50% by volume of glass fiber, and 40 to 80% by weight of polytetrafluoroethylene, in which the open portions of the expanded metal are filled with the matrix. The expanded metal is coated with the matrix so that a friction sliding layer composed of the matrix is formed at a thickness of 0.01 to 0.3 mm. The matrix also contains 10 to 50% by volume of zinc sulfide or barium sulfate having a particle size range of 0.1 to 1.0 .mu.m (Japanese Patent Application Laid-open (KOKAI) No. 57919/1988 corresponding to U.S. Pat. No. 4,847,135),
(6) a slide bearing including a rigid backing pad having a face, a metal mesh covering the face of the pad, means fixedly securing the mesh to the pad, and a sheet or low friction bearing material overlaying the mesh and having a portion of its thickness pressed into the mesh to interlock the bearing material with the mesh (U.S. Pat. No. 4,238,137),
(7) a sheet material for sliding surface bearings, consisting of a network of expanded metal coated with a fluorine-containing polymer, having a network of expanded metal, which consists of a wrought aluminum alloy having an elongation at break .delta..sub.5 =8 to 20% and a Brinell hardness HB 35 to 65 and which has a coating of a blend of 5 to 25% by volume lead, 10 to 50% by volume glass fibers, and 40 to 80% by volume polytetrafluoroethylene (European Patent No. 40448B), and
(8) a sheet material for sliding surface bearings made of an expanded metal mesh composed of a wrought aluminum alloy having an elongation at break .delta..sub.5 =8 to 20% and a Brinell Hardness Number HB of 35 to 65; a primer layer coating the surface of the expanded metal mesh having a thickness of 2 to 10 micrometers; and on the primer layer another layer composed of 10 to 30 wt. % of a filler for improving the thermal conductivity and wear resistance, 10 to 30 wt. % glass fibers and 40 to 80 wt. % polytetrafluoroethylene (PTFE) (U.S. Pat. No. 4,624,887).
Conventionally, in order to improve the wear resistance and the creep resistance previous experiments have added various fillers such as glass fiber, glass beads, carbon fiber, carbon powder and the like.
However, since fillers such as glass fiber and the like are hard, these fillers sometimes damage the opposite member and further the PTFE itself is cut and removed, so that there is a risk of causing abrasive wearing. As a result, depending on the amount of the filler, the wearing amount tends to increase.
Also, the sliding member with a metal network structure as a base member becomes sufficiently intimate with the surface of the opposite member owing to the flexibility generated in its thickness direction. This avoids creep and permits rotation of the opposite member which remains smoothly supported.
Depending on the use of the sliding member, electrical conductivity may be required for the sliding member.
Applicants have found that by adding glass fiber, glass powder, carbon fiber and/or carbon powder as a filler, and a phosphate to a fluororesin such as polytetrafluoroethylene, mixing them homogeneously to obtain a fluororesin composition, pressing and molding it at a normal temperature at 300 to 600 kg/cm.sup.2, and baking (heating) it under normal pressure at 360.degree. to 380.degree. C., the thus obtained sliding member exhibits a stable performance with a low friction coefficient showing no damage to an opposite member at all during sliding and is electrically conductive.